In the following description, the term “underwater pipeline” is intended to mean a pipeline laid on the sea bed. Though specific reference is sometimes made in the description to laying underwater pipelines in the sea, it is understood that the object of the present invention applies to any stretch of water large and deep enough to employ laying vessels, and is in no way limited to marine applications.
A known guide device, such as the one described in document U.S. Pat. No. 4,030,311, is fitted to a laying ramp, and comprises a cradle housing the pipeline; a frame fixed to the laying ramp; and a spacer mechanism located between the first frame and the cradle to selectively adjust the distance between the cradle and the frame.
In document U.S. Pat. No. 4,030,311, the spacer mechanism comprises a double-acting hydraulic cylinder; and an arm hinged to the frame and connected to the hydraulic cylinder.
The cradle is hinged to the arm by a double hinge to swing freely about an axis crosswise to the feed path, and about a further axis perpendicular to said axis and to the feed path.
The guide device in U.S. Pat. No. 4,030,311 has the advantage of enabling fast adjustment of the distance between the frame and cradle, and enabling the cradle to adjust automatically to the position of the pipeline at the cradle.
On the other hand, the guide device in U.S. Pat. No. 4,030,311 fails to prevent the pipeline from transmitting severe stress to and possibly impairing operation of the guide device, and vice versa, fails to prevent the guide device from transmitting severe stress to and so causing structural damage to the pipeline just as it is being laid.
Moreover, from the construction standpoint, some parts of the guide device, subjected to and therefore designed to withstand particularly severe stress, are extremely bulky and heavy, thus resulting in an extremely heavy laying ramp that is difficult to manoeuvre.
For a clearer understanding of the problems involved, it is important to bear in mind that the guide device is fitted to a laying ramp, in turn installed on a laying vessel.
A laying vessel is a floating unit comprising a pipeline assembly line or so-called “firing line”, and equipped at the stern with the laying ramp, which defines an extension of the firing line and serves to ease the pipeline onto the sea bed as the floating unit advances.
The method of producing and laying the pipeline comprises assembling the pipeline on a substantially horizontal firing line; and laying the pipeline using the laying ramp, which, in a work configuration, guides and supports the pipeline along an arc-shaped path partly above and partly below sea level. Using this method, the pipeline being laid assumes an “S” shape between the vessel and sea bed, with curves whose radius of curvature depends on the rigidity of the pipeline, and various types of vessels can be employed, such as pontoons, barges, semisubmersible and single-hull vessels. Pontoons and barges are the underwater-pipeline laying vessels most commonly used in the past; semisubmersible vessels are particularly suitable for laying underwater pipelines in bad weather (rough sea); and single-hull vessels have the advantage of being fast, have ample pipe storage space, and can accommodate relatively long firing lines with a high output rate.
Laying underwater pipelines, however, calls for relatively good weather conditions, so much so that, in rough-sea conditions, laying is suspended: the completed part of the pipeline is left on the sea bed and attached to a winch cable for retrieval later; and the laying ramp is set as high as possible above sea level in a so-called “survival configuration”.
The laying ramp normally comprises an inner ramp hinged to the vessel, and an outer ramp hinged to the inner ramp, and can assume various work configurations, depending on the depth of the sea bed and the characteristics of the pipeline; an in-transit configuration, in which the inner and outer ramps allow unobstructed movement of the vessel; and said “survival configuration”. Accordingly, the laying ramp is connected to the vessel by an actuating assembly as described, for example, in the Applicant's U.S. Pat. No. 4,257,718.
To guide and feed the pipeline forward, the laying ramp is equipped with a number of guide devices aligned to define the feed path of the pipeline.
In connection with the above, it should be pointed out that the vessel and the pipeline are subjected to stress by the sea during the laying operation. On the one hand, the vessel, though maintained in position and jogged along a given course by a system of mooring lines or by so-called dynamic positioning using thrusters, is still subject to undesired movements caused by waves and sea currents.
On the other hand, the pipeline is also subjected to the same waves and currents. But, because of the difference in the shape and mass of the vessel and the pipeline, and the fact that the pipeline is substantially flexible and rests partly on the sea bed, the waves and sea currents produce additional stress between the laying ramp and pipeline.
This stress may result in severe loads being transmitted between the guide devices and the pipeline, and may impair the structural integrity of the guide devices, of the laying ramp as a whole, and of the pipeline.
To eliminate this drawback, Patent Application WO 96/08605 describes a guide device of the above type comprising two cradles hinged to an intermediate frame, in turn connected to a further intermediate frame by a number of air springs.
Over the years, this guide device has proved particularly effective in safeguarding the structure of the device itself. However, known guide devices still call for a good deal of maintenance, on account of the wear to which the components are subject.